I agree with J, you definitely should drive the GT350 first as much as possible, I had one a few years ago at the same time as my 12 GT500 and I still have the GT500 while the 350 only lasted me a few months before i ditched it, one of my least favorite cars to drive, looks awesome and handles...
are you picking up satin or chrome AFS 18"? If your looking for satin a have a set of basically brand new ones with Michelin PSS 4S tires im selling in NJ for $1500
I have arguably the most desirable combo 2012 GT500 (grabber blue, fully loaded navi, glass roof, recaros, track pack) with roughly 15k miles which seems to be around the low $50k area in the this market, even the least desirable black base model 13/14 is gonna bring 10% ish more then my car...